Ferroelectric Electroresistance after a Breakdown in Epitaxial Hf0.5Zr0.5O2 Tunnel Junctions

The recent discovery of ferroelectricity in doped HfO2 has opened perspectives on the development of memristors based on ferroelectric switching, including ferroelectric tunnel junctions. In these devices, conductive channels are formed in a similar manner to junctions based on nonferroelectric oxides. The formation of the conductive channels does not preclude the presence of ferroelectric switching, but little is known about the device ferroelectric properties after conduction path formation or their impact on the electric modulation of the resistance state. Here, we show that ferroelectricity and related sizable electroresistance are observed in pristine 4.6 nm epitaxial Hf0.5Zr0.5O2 (HZO) tunnel junctions grown on Si. After a soft breakdown induced by the application of suitable voltage, the resistance decreases by about five orders of magnitude, but signatures of ferroelectricity and electroresistance are still observed. Impedance spectroscopy allows us to conclude that the effective ferroelectric device area after the breakdown is reduced, most likely by the formation of conducting paths at the edge.


Supporting Information S3
Resistance data was recorded as a function of writing voltage respectively for HRS and LRS for samples with different HZO thickness ( Figure S3). This measurement was conducted by prepoling samples following a sequence: Vw = ±1 V, ±2 V, ± 2.5V, etc. For samples of t = 4.6nm and t = 3.6nm, this Vth is roughly around ± 4.5V; however, for the thinnest sample t = 2.2nm, Vth ≈ ±7V. The tendency of increasing threshold voltage with HZO thickness is consistent with the information shown in Figure S3. S-3

Supporting Information S4
R(Vw) loops before and after the soft-breakdown are shown for samples of t = 3.6 nm and 2.2nm in Figure S4(a,b), respectively. The ER in the sample t = 3.6nm changes from ER = 26% to ER = 9.8% [ Figure S4(a)], while in sample t = 2.2nm it changes from ER = 33% to ER = 1.6% [ Figure S4  S-4

Supporting Information S5
The HRS and LRS states dependence on w is shown in Figure S5

Supporting Information S6
We now try to disclose the conduction mechanisms in the explored system. Figure S6 V characteristics analysis. In Figure S6(b), the I-V curves after prepoling the sample with + or -5 V (as indicated) are shown. It can be observed that the curves show sigmoidal shape. Lines through data points correspond to the Brinkman model fitting. 14 The obtained parameters are summarized in see Supporting Information Table S1, which show reasonable agreement with the literature. 5,9,15 In Figure S6(c), the I-V curves after prepoling the sample with + or -5 V after breakdown are shown. It can be observed that the contrast between both curves is smaller in agreement with the observed smaller ER. After breakdown, it can be observed larger conductivity, which indicates the presence of a metallic-like channel. In fact, Brinkman model fitting results in smaller barrier height and thickness, as expected by the fact that the conductive channel would cause an extrinsic reduction of these parameters (Table S1). The obtained parameters from data fitting are nearer to the expected ones, if a parallel resistance is considered (Table S1). S-6 Table S1. Barrier parameters extracted from fittings are shown in Figure. 4

Supporting Information S7
Capacitance (C-f) and impedance (Z''-Z') measurement conducted in sample with different HZO thickness.
Similar behavior is observed for all samples irrespectively of their thickness. cycled, a ringed wrinkle was observed from its topography, as shown in the region between two dashed circles in (c), which indicates to a volume expansion caused by filament formation. And the piezo response in this area is considerably lower as shown in its amplitude (d).